This invention relates to discharge vessels and high intensity discharge lamps having such discharge vessel.
Usually, the arc chamber in a discharge vessel of a high intensity discharge (HID) lamp has ellipsoidal or cylindrical shape, and sometimes the end part of the arc chamber is also shaped, e.g. to have a hemispherical or conical geometry to either reduce end losses, or optimize arc chamber thermals. In a horizontally operated HID arc chamber, the convection gas flow that is induced by the buoyancy force acting upon a hot gas volume surrounded by a cooler gas environment makes the arc to bend upwards. This is, because a convection cell is developing in the arc chamber. The fill gas convection in the central part of the convection cell pushes the hot gas to reach the upper wall subsequently the hot gas turns towards the two end parts of the chamber. On the other hand, the cooler gas arriving from the end parts of the arc chamber and flowing at the bottom of the chamber turns to move upwards in the central section of the convection cell. The gas convection in this way modifies the temperature distribution of the plasma to become non rotational symmetric in the arc chamber. As a result, this leads to arc bending upwards, and consequently the arc shape is distorted from a straight line into an upward curved line.
One can conclude then, that the conventional geometry options of the HID arc chambers do not fully control the shape of the arc, namely the degree of arc bending when the lamps are burning in horizontal orientation. The conventional arc chambers by their relatively large dimensions act as a single convection cell that allow gas velocities to be extremely intense, and lead to the above described phenomenon of arc bending.
There have been some attempts so far to make the arc straighter. Some of the current HID lamps, among others discharge automotive lamps, still use “constant wall thickness” geometry, that is an ellipsoidal discharge vessel and an ellipsoidal inner arc chamber geometry, but the leading lamps in the market have discharge vessels of a more complex shape. The most common shape is an ellipsoidal outer geometry, and a cylindrical central portion plus conical end portions inside. The aim of the central cylindrical portion is to make the arc “wall stabilized” that is to “push” the bowed arc towards the longitudinal axis of the arc chamber.
Additionally, proposals for making the shape of the inner arc chamber partly convex can also be found in the patent literature. Either the bottom or the top center portion of the inside geometry is made to be convex in these proposals. In addition to the noble gas fill, arc chambers of HID lamps generally also contain other ionizable fill materials that are in liquid phase when the lamp is in operation. At metal halide lamps, this liquid gas phase constitutes a halide pool located at the coldest portion of the arc chamber. The liquid phase is in equilibrium with its vapor. When the inner surface is convex at the bottom, the aim is to raise the position of the liquid halide pool so that it becomes closer to the arc, and the vapor pressure of the halide dose is increased due to the increased dose pool temperature by more effective radiation heating from the arc. In this respect, U.S. Pat. No. 7,348,731, for example, discloses a high-pressure gas discharge lamp with an asymmetrical discharge space (arc chamber) and/or an asymmetrical discharge vessel. The arc chamber (discharge space) has a volume, which is reduced by a given first factor in comparison with the volume of the arc chamber of known mercury-containing discharge lamps. The quantity of the light-generating substances in the arc chamber (discharge space) is reduced by the same factor in the simplest case, or even more strongly in less simple cases. This avoids the risk of an impairment of the imaging properties of the lamp due to non-evaporated light-generating substances, which may shade off a portion of the luminous discharge arc and/or the tips of the electrodes.
The problem of the present invention is however different, namely to make the arc of a horizontally operated HID lamp straighter. The straightness of the arc between the opposing electrodes has great advantage in high efficiency optical systems, since imaging of a straight arc is more efficient than that of a distorted or bent one. More importantly, the straightness of the arc is a need in automotive headlamps where strict requirements exist with respect to the maximum and minimum illumination levels on the road or the test screen. The straighter the arc, the easier to meet these requirements.
Accordingly, there is a need for a HID lamp with an improved discharge vessel configuration, which provides better discharge arc orientation within the arc chamber in horizontal operational position. There is also a need for an improved discharge vessel structure, which ensures that the light distribution of the lamp, such as an automotive HID lamp, will be more homogenous in the illumination space. It is sought to provide a solution, which, besides having an improved discharge arc orientation, applicable to discharge vessels of either fused silica glass or ceramic material.